I have an iMac (2017) and a Mac Mini, both running high Sierra 10.3.4. When I try to access the Mini's screen from the iMac, it works at first, but if I maximize the window so that the shared screen takes up my whole screen, the mouse pointer disappears and keyboard control stops working. If I switch back to a regular window, it still doesn't work. If I share in the other direction (i.e., accessing the iMac's screen from the Mini), it works fine. Any idea what might be causing this?